1. Field of the Invention
The present invention relates to a centrifugal casting apparatus suitable for precision casting of jewelry, dental articles, artistic handicraft, precision machine parts or the like.
2. Related Background Art
The centrifugal casting apparatus for precision casting is provided with a horizontal rotating arm connected to the upper part of a vertical rotating shaft, the arm being provided, at an end thereof with respect to a rotational center thereof, with a crucible, a receiver for mounting the crucible and a mold positioned at the external periphery side and, at the other end, with a balancing weight. The molten metal in the crucible is poured into the mold to effect casting by the centrifugal force generated by the rotation of the rotating shaft in the centrifugal casting apparatus. For melting the metal in the crucible, there is generally employed high frequency induction heating, by providing a coil around the crucible and applying a current to the coil, thereby inducing high frequency in the metal to achieve heating thereof.
The metal employed in precision casting can be an elementary metal or an alloy, such as gold, silver, platinum, paradium, rhodium, nickel, white gold, copper alloys, white metal, carbon steel, stainless steel, cast iron, heat-resistant steel, wear-resistant alloy, nickel alloys or aluminum alloys, but these examples are not limitative.
The mold is prepared, for example, by the known lost wax process.
FIG. 9 shows a perspective view of the connecting portion between the rotating shaft and the rotating arm in the conventional centrifugal casting apparatus. As shown therein, the rotating arm 52 is provided with a fixing member 52a for connection with the rotating shaft 51, a balance-side arm 53 provided at an end thereof, and at the other end, with a pair of mold-side arms 54a, 54b connected to both ends of a flat plate 57 integrally formed with the fixing member 52a. A threaded portion 51a at the upper part of the rotating shaft 51 penetrates a hole 52b provided in the fixing member 52a of the rotating arm 52 and is screwed to a fixing nut 55, whereby the rotating shaft 51 is fixed to the rotating arm 52.
Prior to actual centrifugal casting, the rotating arm has to be balanced in weight, since otherwise vibration is induced in the rotating arm during rotation thereof, thus adversely affecting the quality of casting and rendering the casting impossible in extreme case. Thus, balancing of the rotating arm is important. For this reason, prior to fixing the rotating arm 52 to the rotating shaft 51, the balance of the rotating arm 52 is adjusted while it is in an unfixed free state as shown in FIG. 9. That is, a weight (indicated by numeral 2 in FIG. 1) provided on the balance-side arm 53, is adjusted in the horizontal position in such a manner that the rotating arm 52 is balanced at a lower groove 52c of the fixing member 52a, on a pin 56 radially penetrating the threaded portion 51a of the rotating shaft 51 and serving as a fulcrum.
After the rotating arm 52 is balanced as explained above, it is fixed to the rotating shaft 51. Subsequently the metal in the crucible (indicated by numeral 3 in FIG. 1) provided on the mold-side arms 54a, 54b is melted by the high frequency induction coil, then the coil is kept away from the crucible, and the rotating arm is rotated to pour the molten metal in the crucible into the mold (indicated by numeral 4 in FIG. 1). In this operation, the molten metal in the crucible starts to cool, because the supply of heat has already been removed, and is relatively rapidly cooled depending on the kind of the metal. If the temperature of the molten metal to be poured into the mold becomes too low by the cooling, there tends to result incomplete filling or cracks in the casting. It is therefore necessary to pour the molten metal from the crucible to the mold as quickly as possible after the termination of heat supply, and to rotate the rotating arm at least at more than a predetermined speed in order to obtain at least a predetermined centrifugal force.
In consideration of the foregoing, how to promptly cause the rotating arm to reach the predetermined speed is an important factor for the casting quality. FIG. 10 shows the relationship between the rotating speed .omega. and the time T. The rotating speed, at which the molten metal jumps out of the crucible by the centrifugal force is assumed as .omega..sub.n. According to a solid-lined speed curve A in FIG. 10, the time from the start of rotation to the casting can be reduced by (T.sub.2 -T.sub.1) in comparison with a chain-lined speed curve B. The speed curve A is preferable for the casting quality, since it can reach the rotating speed .omega..sub.n faster and can thus reduce the casting time in comparison with the speed curve B.
However, acceleration to the high-speed of the rotating arm as shown in the speed curve A inevitably involves a rapid variation in the speed, and leads to the following drawbacks. As shown in FIG. 10, the rotating arm is subjected to acceleration by rapid speed increase in an area M, and is subjected to an inverse force by deceleration in an area N, so that in the conventional apparatus shown in FIG. 9, the fixing nut 55 for fixing the rotating arm 52 to the rotating shaft 51 is given a rotating torque and is loosened during the rotation. The tightening force of the fixing nut 55 cannot be made strong enough since the force is transmitted through the pin 56, and the contact area between the pin 56 and the groove 52c of the fixing member 52a of the rotating arm 52 is limited and can only provide a limited friction force, thus the nut 55 is easily loosened. For this reason, the rotating arm cannot perform proper rotation, giving undesirable influence on the casting.
It is also conceived to increase the diameter of the pin 56 in order to increase the tightening force or the frictional force mentioned above, but such increased diameter of the pin 56 leads to an increased size of the rotating shaft, with an increased weight thereof, which is unfavorable for the acceleration of the rotating arm.